Sources of UHECR are still a mystery, we do not know which kind of astrophysical object is responsible for the production of these particles. There are basically two different classes of astrophysical mechanisms that could be invoked to accelerate UHECR [1]}, [2]}, [3]}, [4]}. The first class is based on the transfer of energy from a macroscopic object (that can move relativistically or not) through repeated interactions of particles with magnetic inhomogeneities; belongs to this class the diffusive shock acceleration mechanism. The second class is based on the interaction with electric fields that, through high voltage drops, can accelerate particles (at once) until the highest energies; belongs to this class the unipolar induction mechanism in fast spinning neutron stars [1]}, [2]}, [3]}, [4]}.
